1. Technical Field
This invention relates to vibrators generally and, in particular, to pneumatic vibrators in which a free piston is reciprocated by air pressure acting alternately on the ends of the piston. In the subject type of air vibrators, no external controls are required for reciprocal movement of the free piston. Vibrators are used extensively as flow aid devices for bulk material handling, and may be used as core samplers, drives for feeders, screening machines, electrostatic precipitator rapping, galvanizing, and for similar purposes.
2. Background information
Such vibrators include a cylindrical housing having inlet and exhaust ports and a free piston with a passage that connects the inlet port to the portion of the housing between one end of the piston and the end of the housing when the piston is near that end and another passage that similarly connects the portion of the housing at the other end. The piston is reciprocated by pressurized air as the passages in the piston are connected alternately to the inlet in the housing. As the piston reciprocates, it creates vibrations in the housing and in whatever structure the housing is connected.
The piston may carry seals such as piston rings. Other types of vibrators depend instead on a close fit between the piston and the bore of the housing to provide a sufficient restriction to the flow of air between the piston and the cylinder for the proper operation of the vibrator. With time, however, wear occurs between the piston and the bore of the housing such that the clearance between the piston and housing increases, eventually reaching the point where the vibrator does not operate satisfactorily.
In the use of prior art air vibrators, their operating life is usually measured in short periods of time ranging from 2 to 3 months at most. Frequently, abrasive dust is sucked into their exhaust ports due to a temporary vacuum effect when the vibrators are cycled off and pressurized air delivery is discontinued. This condition causes excessive wear internally of the vibrators such that their reciprocation frequently does not start when pressurized air delivery is turned on. In addition, lubrication of the piston must be maintained which is difficult in severe high-temperature environments. Elliptical wear of the facing piston and housing surfaces is usually observed which shortens vibrator life. Also the use of lubricants is highly undesirable where the vibrators are used in areas where sources of fires or explosions must be avoided.
Restoration of vibrators is possible, but only by remachining the cylinder housing and installing a new piston machined to allowable tolerances for the required clearance. This process requires dismounting, shipment to a qualified repair facility, disassembly of the vibrator, machining, reassembly, shipment back to the user, and remounting. In many applications, this procedure is cost prohibitive. In addition, a restored vibrator still experiences the same wear problems. Normally it is uneconomical to rebuild the vibrator a second time.
Several variations have been developed as improvements over the general piston vibrator design. One such design incorporates a removable inner sleeve, which eliminates the need for remachining the housing during the restoration process, thereby permitting the housing to be used indefinitely. These sleeves are designed in such a way that they are inserted into the housing in a close fitting relationship with the housing and must be properly oriented to properly align the inlet and outlet ports. These design limitations necessitate restoration be performed by a qualified repair facility.
Other variations over the general piston vibrator design are offered to improve wear characteristics. These include coating the sleeve or housing with special low-friction type coatings, case hardening the sleeve or housing, and inserting lubrication fluid into the air line. Many types of low-friction coatings can be used, but the trade-off between lower friction at higher costs makes most low-friction coatings economically unfeasible. Case hardening does improve wear characteristics, but the continuous metal-to-metal contact eventually causes failure. Lubricators offer a reasonably inexpensive method for reducing friction, but the periodic maintenance required to insure the vibrators receive adequate lubrication is undesirable. Also, oil-laden air which can form explosive mixtures must be avoided in certain applications such as in power plant operations.